This invention relates to a method and apparatus for thermally tempering glass sheets which are to be subsequently laminated. In particular, it relates to the problem of heating glass sheets of different thicknesses as part of a tempering process while maintaining their surface contours closely matched for subsequent lamination. The sheets are usually bent to their desired shapes in a separate operation prior to tempering.
The use of tempered or laminated glass is widespread in applications were strength and/or safety are of concern. In certain specialized applications, most typically for aircraft glazing, it is desired to fabricate a glass transparency that is both tempered and laminated. In such a case, it is necessary to temper the glass sheets individually and then laminate two or more together with a layer of plastic between each pair of adjacent glass surfaces. Because the thermal tempering process involves heating the glass sheets to a temperature at which slight changes in the shape of the glass can take place, great care must be taken to insure that a pair of glass sheets to be laminated are subjected to nearly identical temperature conditions during tempering so that the curvatures of their mating surfaces remain in close correspondence. Mismatched surfaces can lead to poor quality laminates. In addition, when curved glass sheets having mismatched surfaces are laminated together, lamination imparts to the mismatched portions stresses likely to cause delamination and even breakage of the laminated transparency during service.
It has now been proposed to laminate together tempered glass sheet of differing thicknesses. But glass sheets of different thicknesses will heat up at different rates under a given set of furnace operating conditions, and therefore their surface contours will tend to diverge from conformity with one another in the tempering process. It is generally not practical, when using conventional massive furnaces used in mass production operations, to alter furnace conditions from one piece to another because of the long time lag usually entailed in altering the furnace conditions. Nor has it been found acceptable to heat thick pieces and thin pieces in separate groups on the same furnace or two different furnaces because of the difficulty of duplicating thermal conditions in two separate furnace campaigns.
Lightweight furnaces have been employed for heating glass sheets in the prior art, such as in U.S. Pat. No. 3,697,243 to Artama and U.S. Pat. No. 3,762,905 to Artama et al., but how such furnaces might be deployed to solve the problem of tempering glass sheets of unequal thickness to be laminated has not been addressed heretofore.
Another substantial body of prior art discloses various glass heating methods involving two stages or more. U.S. Pat. Nos. 3,914,119 and 3,951,634 both disclose methods of reheating glass sheets after bending and prior to tempering, but all the sheets in a given series are of the same thickness and are treated in the same way. U.S. Pat. Nos. 2,198,622; 2,244,113; 3,223,499; 3,406,022; and 3,809,544 disclose various glass heat-treating processes in which the glass articles in each are subjected to different rates of heating at different stages of the process. None deals with the problem of tempering differing thicknesses of glass. U.S. Pat. No. 3,827,872 discloses preheating selected areas of a glass sheet about to be heated and tempered.